Method for the production of aqueous polymer dispersions containing very few residual monomers and use thereof

ABSTRACT

The invention relates to a method for reducing the amount of residual monomers in aqueous polymer dispersions means of chemical post-treatment. Post-treatment in the aqueous polymer dispersion is carried out by adding a redox system which contains a) 0.005-5 wt. % of an oxidation agent which contains an organic peroxide. and b) 0.005-5 wt. % of a reduction agent which contains sulfinic acids or salts thereof. Additionally. the redox system can, optionally, contain catalytic amounts of a polyvalent metallic ion which can be treated in several valent stages. Post-treatment can be carried out at a temperature ranging from 20-100° C. and at a PH-value ranging from 2-9. The invention also relates to the use of the inventive post-treated polymer dispersion for producing adhesives, coarings, powders, constructive chemical products or for refining textiles or paper.

The invention relates to a process for reducing the amount of residualmonomers in aqueous polymer dispersions by chemical aftertreatment.

After their preparation by free radical emulsion polymerization orcopolymerization, aqueous polymer dispersions, in addition to having apolymer solids content of from 30 to 75% by weight, also contain anundesired proportion of unpolymerized free monomers (“residualmonomers”) and may contain further volatile nonaqueous components, owingto the incomplete polymerization of the monomers used in the freeradical main polymerization, which is generally carried out to a monomerconversion of 95 and preferably from 98 to 99% by weight. For ecologicalreasons, in the case of adhesive, coating and building products, themarket requires the provision of aqueous polymer systems which have aslow a content as possible of residual monomers, which contributes to thetotal VOC content of the dispersion (VOC=volatile organic compounds), incombination with the same processing and application properties.

In the case of physical methods, such as, for example, “stripping”, inwhich gaseous nitrogen is passed through the dispersion, or, forexample, steam distillation, there is the danger of the formation ofcoagulum and deposits. Moreover, these methods are often associated witha considerable time consumption and costs and are thereforeuneconomical. In addition, particularly dispersions having a relativelyhigh viscosity are known to be very difficult to demonomerizephysically.

In addition to physical methods, various chemical methods for reducingresidual monomer contents of aqueous polymer dispersions are available.These often include a stage downstream of the actual emulsionpolymerization, by means of special redox initiator systems. Adisadvantage known to a person skilled in the art is the decrease inviscosity which is associated with the chemical demonomerization,especially in the case of highly viscous dispersions, which decrease isoften undesired.

Systems for chemical demonomerization are to be found in the patentliterature.

Thus, for example, DE-A 198 39 199 discloses the reduction of the amountof residual monomers in aqueous polymer dispersions by aftertreatmentwith a hydroperoxide and/or a compound liberating hydrogen peroxide, analdehyde R²CHO (e.g. acetaldehyde) in combination with an inorganicdithionite (e.g. sodium dithionite) and catalytic amounts of apolyvalent metal ion, e.g. salts of Fe²⁺. In the redox system, hydrogenperoxide is preferably used as the oxidizing agent,,but also potassiumperoxide, sodium peroxide, and further precursors forming hydrogenperoxide in an aqueous medium. The use of organic hydroperoxides is alsomentioned.

DE-A 199 42 776 of the same Applicant describes a process for reducingthe residual monomer content in aqueous dispersions by aftertreatmentwith a hydroperoxide or a compound liberating hydrogen peroxide and acarbonyl compound (R²(C═O)R³ in which R²═H, alkyl, etc. and R³═—CO₂M,SO₃M, etc.) in combination with an inorganic dithionite and catalyticamounts of a polyvalent metal ion, e.g. salts of Fe²⁺.

DE-A 197 43 759 presents the preparation and use of a novel class ofsulfinic acid derivatives. They have the following structure:MO—SO—CR¹R²R³ in which M=Na, K, NH₄, Mg, Ca, Zn, R^(1═OH, NH) ₂,R²═COOM, COOR, CONR₂, R³═H, alkyl, alkenyl, cycloalkyl, aryl. The use ofthe sulfinic acid derivatives as a cocatalyst in the emulsionpolymerization is claimed. The object of this use is to provide sulfinicacid derivatives whose chemical properties are as similar as possible tothose of the formaldehyde sulfoxylates, which however eliminate noformaldehyde during and after use. This publication provides noindications that very low residual monomer contents are to be expectedwith the reducing agents according to the invention when used in a redoxaftertreatment. Research Disclosure 1983/November 2000 (439062) presentssulfinic acid derivatives for emulsion polymerization and especially forthe aftertreatment for reducing the residual monomer contents. Thesereducing agents are proposed for providing formaldehyde-free dispersionswhich have little discoloration. They have the following structure:MO—SO—CR¹R²R³ in which M=Na, K, NH₄, Mg, Ca, Zn, R¹═OH, NH₂, R²═COOM,COOR, CONR₂, R³═H, alkyl, alkenyl, cycloalkyl, aryl. Oxidationcomponents recommended are hydrogen peroxide, persulfate, tert-butylhydroperoxide or di-tert-butyl peroxide or combinations thereof. Inaddition, a metal salt (Fe, etc.) perketals and percarbonates are notmentioned.

EP-A-1 201 685 describes the use of a redox initiator system for thepreparation of dispersions having a low formaldehyde content, containingcopolymers of vinyl acetate and N-methylolacrylamide. As reducing agentsare compounds of the structure MO(SO)—CR¹R²R³ in which M=H, NH₄,monovalent metal ion, R¹═OH or NR⁴R⁵, in which R⁴ and R⁵ are each H orC₁-C₆-alkyl, R²═H, alkyl, alkenyl, cycloalkyl, aryl, R³═CO₂M. There areno indications of low residual monomer contents with the use of thereducing agents according to the invention when employed in a redoxaftertreatment.

EP-A-1 199 315 describes the use of a redox system for the preparationand aftertreatment of polymer dispersions comprising a mixture of awater-soluble and a water-insoluble oxidation component and a sulfinicacid or a salt thereof. The examples teach that the sole use of awater-insoluble oxidizing agent in combination with the sulfinic acidderivative (Comp. Ex. B) leads to poorer residual monomer and VOCcontents.

EP-A-1 199 316 describes the use of a non-formaldehyde-forming freeradical redox initiator system for the preparation and aftertreatment ofpolymer dispersions, which system comprises tert-alkyl hydroperoxide,tert-alkyl peroxide or tert-alkyl perester, the tert-alkyl group havingat least 5 carbon atoms, and a “non-formaldehyde-liberating” reductioncomponent, such as isoascorbic acid, sodium metabisulfite, sodiumbisulfite, sodium dithionite and sodium 2-hydroxy-2-sulfinatoacetate.The advantageous effect with regard to the formaldehyde content arisesfrom the use of tert-alkyl peresters as oxidizing agents which containtert-amyl instead of tert-butyl groups as the alkyl group.

Advantageous effects regarding the particular efficiency of the residualmonomer reduction with the mentioned combinations of oxidation andreduction components are not disclosed. In addition, the raw materialcosts for oxidizing agents containing tert-amyl groups are substantiallyhigher than those of the corresponding tert-butyl compounds.

WO 00/22003 describes a process for emulsion polymerization in which apolymerization initiator which is not a hydroperoxide is combined with areducing agent in order to achieve a reduction of the duration of theprocess in an initial cold start-temperature process. Reducing agentssuch as sodium formaldehyde sulfoxylate, ascorbic acid, sodiumbisulfite, sodium metabisulfite and sodium dithionite are used. Inaddition, sugar and aldehydes, such as glutaraldehyde, are alsorecommended as reduction components. The particularly preferred reducingagent is sodium formaldehyde sulfoxylate. There are no indicationsregarding freedom from formaldehyde, tendency to discoloration (use ofascorbic acid) or a particular efficiency of the residual monomerreduction with the mentioned combinations of oxidation and reductioncomponents. However, this process is not the aftertreatment of aprepared polymer dispersion but the use of the redox system as aninitiator for initiating the emulsion polymerization.

U.S. Pat. No. 5,886,140 describes a process for reducing the residualmonomer content in aqueous dispersions by chemical aftertreatment with aredox system, the reducing agents used being complexes of zincsulfoxylate and formol (formalin), reducing sugars or acid derivativesthereof and C₅-C₄-carboxylic acids. Oxidation components used arehydroperoxides, peroxides, peroxydicarbonates or peroxyesters andselected mixtures of these components with inorganic persulfates. Theuse of formaldehyde-free sulfinic acid derivatives as reducing agents isnot mentioned. None of the publications cited indicates that dispersionsexhibiting little yellowing in combination with particularly lowresidual monomer contents are obtainable with the redox systemsdescribed there, by means of redox agents which lead to no additionalintroduction of formaldehyde into the dispersion. In particular, thereis no information regarding the suitability of redox systems describedfor the demonomerization of dispersions having a relatively highviscosity.

It was therefore the object of the present invention to provide a noveland effective process for the particularly efficient reduction ofresidual monomer contents in aqueous polymer dispersions, it beingintended to minimize the known disadvantages associated therewith, suchas yellowing, formaldehyde introduction by the aftertreatment and theformation of coagulum. Moreover, the invention should be easy to useindustrially and should also be capable of being applied to dispersionshaving a relatively high viscosity without a serious decrease inviscosity.

It was surprising that this object is achieved by a process for reducingthe amount of residual monomers in aqueous polymer dispersions, in whichan aftertreatment of the aqueous polymer dispersions containing residualmonomers is carried out with addition of a redox system which comprises

-   -   a) from 0.005 to 5% by weight, based on the total weight of all        monomers used for the preparation of the polymer dispersion, of        at least one oxidizing agent based on an organic peroxide from        the class consisting of the    -   a1) peresters, characterized by the structure        R¹—CO—O—O—R²,   (1a)        in which R¹, R²=alkyl, aryl, cycloalkyl, aralkyl, R³CO, R³OCO in        which R³=alkyl, aralkyl, aryl and the substituted variants        thereof and/or    -   a2) percarbonates, characterized by the structure        R⁴—O—CO—O—O—R⁵,   (1b)        in which R⁴, R⁵=alkyl, aryl, cycloalkyl, aralkyl, R⁶CO, R⁶OCO in        which R⁶=alkyl, aralkyl, aryl and the substituted variants        thereof, it being possible for the radicals R⁴, R⁵ and R⁶        themselves to contain percarbonates, and/or    -   a3) perketals, having the following structure:        R⁷—O—O—CR⁹R¹⁰—OO—R⁸,   (1c)        in which R⁹, R¹⁰═H or alkyl and R⁷, R⁸=alkyl, aryl, cycloalkyl,        aralkyl, R¹¹CO, R¹¹OCO in which R¹¹=alkyl, aralkyl, aryl and the        substituted variants thereof, and    -   b) from 0.005 to 5% by weight, based on the total weight of all        monomers used for the preparation of the polymer dispersion, of        at least one reducing agent from the group consisting of the        sulfinic acids and the salts thereof having the structure        MO—SO—CR¹²R¹³R¹⁴   (2)        in which M=H, NH₄, a monovalent metal ion or one equivalent of a        divalent metal ion of groups Ia, IIa, IIb, IVa or VIIIb of the        Periodic Table of the Elements, in which R¹²═OH, NR¹⁵R¹⁶ in        which R¹⁵, R¹⁶═H or C₁-C₆-alkyl, in which R¹³═H, an alkyl,        alkenyl, cycloalkyl or aryl group, it being possible for these        groups to have 1, 2 or 3 substituents which, independently of        one another, are selected from C₁-C₆-alkyl, OH, O—C₁-C₆-alkyl,        halogen and CF₃, in which R¹⁴═COOM, SO₃M, COR¹⁵, CONR¹⁵R¹⁶,        COOR¹⁵, in which M, R¹⁵ and R¹⁶ have the abovementioned        meanings, or, if R¹³ is aryl, this may be unsubstituted or        substituted as stated above, and R¹⁴ is also H, and the salts        thereof.

If, in the components al), a2) and a3), any radicals are alkyl, this isa straight-chain or branched alkyl group which preferably has from 1 to22, in particular from 1 to 18, carbon atoms. The alkyl group may beunsubstituted or substituted.

If, in the components al), a2) and a3), any radicals are cycloalkyl,this is a cycloalkyl group having three to eight ring carbon atoms,preferably five to six ring carbon atoms. The cycloalkyl group canlikewise be unsubstituted or substituted.

If, in the components al), a2) and a3), any radicals are aryl, this is amononuclear or polynuclear aromatic radical which typically has five tofourteen, preferably six to ten, ring carbon atoms, in particular phenylor naphthyl. The aryl group can likewise be unsubstituted orsubstituted.

If, in the components al), a2) and a3), any radicals are aralkyl, thisis an aromatic-aliphatic radial which typically has five to fourteen,preferably six to ten, ring carbon atoms and is linked to the remainderof the molecule via an alkyl group, in particular benzyl. The aralkylgroups can likewise be substituted or unsubstituted.

Examples of possible substituents of these radicals are monovalentorganic radicals, such as alkyl, cycloalkyl, aryl, aralkyl, alkoxy,aryloxy, hydroxyl, amino, N-alkylamino, N,N-dialkylamino or halogenatoms.

In addition, the redox system can, if required, also contain catalyticamounts of a polyvalent metal ion which may occur in a plurality ofvalency states.

The oxidizing agent of the redox system for the process according to theinvention should be capable of forming free radicals. In the redoxsystem, preferably the

-   -   a1) peresters are used. Particularly preferably, tert-butyl        perbenzoate, tert-butyl peroxy-3,5,5-trimethylhexanoate and        tert-butyl peroxy-2-ethylhexanoate, very particularly preferably        tert-butyl perbenzoate are used as oxidizing agents, but also        cumyl peroxyneodecanoate, 1,1,3,3-tetramethylbutyl        peroxyneodecanoate, tert- butyl peroxyneodecanoate,        1,1,3,3-tetramethylbutyl peroxypivalate, tert-butyl        peroxyneoheptanoate, tert-amyl peroxypivalate, tert-butyl        peroxypivalate, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate,        tert-amyl peroxy-2-ethylhexanoate, tert-butyl        peroxydiethylacetate, tert-butyl peroxyisobutyrate, tert-butyl        peroxyacetate and tert-amyl peroxybenzoate, or the    -   a2) percarbonates 1-(2-ethylhexanoylperoxy)-1,3-dimethylbutyl        peroxypivalate, di(2-ethylhexyl)peroxydicarbonate,        2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane,        tert-amylperoxy 2-ethylhexyl carbonate, tert-butylperoxy        isopropyl carbonate and tert-butylperoxy 2-ethylhexyl carbonate,        or the    -   a3) perketals        1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane,        2,2-di(4,4-di(tert-butylperoxy)cyclohexyl)propane,        1,1-di(tert-butylperoxy)cyclohexane,        2,2-di(tert-butylperoxy)butane,        3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane can be used.

However, it is also possible to use mixtures of different oxidizingagents. In industrial operation, it is of course possible to use thehydrophobic peroxide components, for reasons of process safety, in theform of an aqueous emulsion which is stabilized with surface-activesubstances or polymeric stabilizers.

The amount of oxidizing agent added is usually in the range of from0.005 to 5% by weight, preferably from 0.02 to 3% by weight,particularly preferably from 0.02 to 2% by weight, very particularlypreferably from 0.05 to 1% by weight, in particular from 0.05 to 0.5% byweight, based on the total weight of all monomers.

The reducing agent of the redox system for the process according to theinvention is described under b). In the description and the claims ofthe present Application, the expressions mentioned below with regard tothe radicals for the reduction component (2) have the followingmeanings:

Alkyl in the compounds of component b) is a straight-chain or branchedalkyl group which preferably has 1 to 6, in particular 1 to 4, carbonatoms. Examples of alkyl groups are methyl, ethyl, n-propyl, isopropyl,n-butyl, tert-butyl, n-hexyl, etc. The same applies to the alkyl groupsin O-alkyl.

Alkenyl in the compounds of component b) is a straight-chain or branchedalkenyl group which preferably has 3 to 8 carbon atoms, in particular 3to 6 carbon atoms. A preferred alkenyl group is, for example, the allylgroup.

Cycloalkyl in the compounds of component b) is in particularC₃-C₆-cycloalkyl, cyclopentyl and cyclohexyl being particularlypreferred.

Aryl in the compounds of the component b) (also in aralkyl) ispreferably phenyl or naphthyl. If the aryl radical is a phenyl group andis substituted, it preferably has two substituents. These are present inparticular in the 2- and/or 4-position.

Halogen is F, Cl, Br and 1, preferably Cl and Br.

M is preferably an ammonium or alkali metal ion or one equivalent of analkaline earth metal or zinc ion.

Suitable alkali metal ions are in particular sodium or potassium ions,and suitable alkaline earth metal ions are especially magnesium andcalcium ions.

-   -   R¹² is preferably a hydroxyl or amino group.    -   R¹³ is preferably a hydrogen atom or an alkyl or aryl group        which may be substituted as above. It preferably has one or two        hydroxyl and/or alkoxy substituents.    -   R¹⁴ is preferably either COOM or COOR¹⁵ (M and R¹⁵ have the        abovementioned meanings) or, if R¹³ is aryl which may be        substituted as stated above, also a hydrogen atom.

A preferred embodiment comprises compounds of the formula (2) in which

-   -   M is an alkali metal ion or one equivalent of an alkaline earth        metal or zinc ion;    -   R¹² is a hydroxyl or amino group;    -   R¹³ is H or alkyl; and    -   R¹⁴ is COOM or COOR¹⁵, in which M is H, an alkali metal ion or        one equivalent of an alkaline earth metal ion and R¹⁵ is        C₁-C₆-alkyl.

A further preferred embodiment comprises compounds of the formula (2) inwhich

-   -   M is an alkali metal ion or one equivalent of an alkaline earth        metal or zinc ion;    -   R¹² is a hydroxyl or amino group;    -   R¹³ is aryl which is unsubstituted or substituted as stated        above, in particular hydroxyphenyl or C₁-C₄-alkoxyphenyl; and    -   R¹⁴ is a hydrogen atom.

Examples of such compounds are:

-   -   2-hydroxyphenylhydroxymethylsulfinic acid or the sodium salt        thereof,    -   4-methoxyphenylhydroxymethylsulfinic acid or the sodium salt        thereof,    -   2-hydroxy-2-sulfinatoacetic acid or the disodium or zinc salt        thereof and    -   2-hydroxy-2-sulfinatopropionic acid or the disodium salt        thereof.

The compounds described above can be used as a pure substance or in theform of technical-grade mixtures. These include, for example, mixturesof the abovementioned sulfinic acid derivatives with, for example,sodium sulfite and/or sulfonic acid derivatives which cannot eliminateany formaldehyde, such as ®Brüggolit FF06 from Brüggemann. The use ofthese compounds is particularly preferred. ®Brüggolit FF06 correspondsto disodium 2-hydroxy-2-sulfinatoacetate or a mixture of disodium2-hydroxy-2-sulfinatoacetate with sodium sulfite and disodium2-hydroxy-2-sulfonatoacetate. A particularly suitable mixing ratio is:disodium 2-hydroxy-2-sulfinatoacetate in an amount of from 50 to 60% byweight, sodium sulfite in an amount of from 30 to 35% by weight anddisodium 2-hydroxy-2-sulfonatoacetate in an amount of from 10 to 15% byweight, based on the total weight of the mixture. However, other mixingratios are also possible in principle.

The amount of reducing agent added is usually in the range from 0.005 to5% by weight, preferably from 0.02 to 3% by weight, particularlypreferably from 0.02 to 2% by weight, very particularly preferably from0.05 to 1% by weight, in particular from 0.05 to 0.5% by weight, basedon the total weight of all monomers. Larger amounts of reducing agentare also possible but are economically less expedient.

The advantageous metal compounds which can be used for theaftertreatment are usually completely soluble in the aqueous medium ofthe polymer dispersion and the metallic components thereof may also becapable of being present in a plurality of valency states. The dissolvedmetal ions have a catalytic action and promote the electron transferreactions between the oxidizing and reducing agents which are actuallyeffective. Suitable dissolved metal ions are in principle iron, copper,manganese, vanadium, nickel, cobalt, titanium, cerium or chromium ions.Of course, it is also possible to use mixtures of different metal ionswhich do not interfere, such as, for example, the system Fe^(2/3+)/VSO₄⁻. Iron ions are preferably used.

The dissolved metal ions are, if required, used in catalytic amounts inthe range from 1 to 1000, preferably from 5 to 500, particularlypreferably from 10 to 120, ppm, based on the total weight of allmonomers.

The process according to the invention is particularly suitable forreducing the amount of residual monomers in aqueous polymer dispersionswhich are obtainable by free radical emulsion polymerization of monomershaving at least one ethylenically unsaturated group.

Monomers having at least one monoethylenically unsaturated group whichare suitable for the process according to the invention include inparticular monomers which can be subjected to free radicalpolymerization in a simple manner, such as, for example,

-   -   aromatic or aliphatic α,β-unsaturated, unsubstituted or        halogen-substituted hydrocarbons (ethene, propene, 1-butene,        2-butene, vinyl chloride, vinylidene chloride, styrene,        α-methylstyrene or o-chlorostyrene, ethene and styrene being        preferred), or    -   esters of vinyl alcohol and monocarboxylic acids having 1 to 18        carbon atoms, such as vinyl acetate, vinyl propionate, vinyl        n-butyrate, vinyl laurate, vinyl stearate and vinyl versatate,    -   esters of α,β-monoethylenically unsaturated mono- and        dicarboxylic acids preferably having from 3 to 6 carbon atoms,        such as, in particular, acrylic acid, methacrylic acid, maleic        acid, fumaric acid and itaconic acid, alkanols having in general        from 1 to 12, preferably from 1 to 8 and in particular from 1 to        4 carbon atoms, such as, in particular, methyl, ethyl, n-butyl,        isobutyl and 2-ethyhexyl acrylate and methacrylate, dimethyl        maleate or di-n-butyl maleate, and nitriles of        α,β-monoethylenically unsaturated carboxylic acids, such as        acrylonitrile, and    -   conjugated C₄₋₈-dienes, such as 1,3-butadiene and isoprene. In        the case of aqueous polymer dispersions produced exclusively by        the method of free radical aqueous emulsion polymerization, said        monomers are as a rule the main monomers which together usually        account for an amount of more than 50% by weight, based on the        total amount of the monomers to be polymerized by the free        radical aqueous emulsion polymerization process. As a rule,        these monomers have only moderate to slight solubility in water        under standard conditions (25° C., 1 atm).

Monomers which have a high water solubility under the abovementionedconditions are, for example, α,β-monoethylenically unsaturated mono- anddicarboxylic acids and the amides thereof, such as, for example, acrylicacid, methacrylic acid, maleic acid, fumaric acid, itaconic acid,acrylamide and methacrylamide, and furthermore vinylsulfonic acid andthe water-soluble salts thereof and N-vinylpyrrolidone.

In the case of aqueous polymer dispersions produced exclusively by themethod of free radical aqueous emulsion polymerization, theabovementioned monomers having high water solubility are usuallyincorporated by polymerization only as modifying monomers in amounts ofless than 50% by weight, as a rule from 0.5 to 20, preferably from 1 to10% by weight, based on the total amount of the monomers to bepolymerized.

Monomers which usually increase the internal strength of the films ofthe aqueous polymer dispersions usually have at least one epoxy,hydroxyl, N-methylol or carbonyl group, or at least two nonconjugatedethylenically unsaturated double bonds. Examples of these areN-alkylolamides of α,β-monoethylenically unsaturated carboxylic acidshaving from 3 to 10 carbon atoms, among which N-methylolacrylamide andN-methylolmethacrylamide are very particularly preferred, and the estersthereof with alkanols having from 1 to 4 carbon atoms. In addition,monomers having two vinyl radicals, monomers having two vinylideneradicals and monomers having two alkenyl radicals are also suitable. Thediesters of dihydric alcohols with α,β-monoethylenically unsaturatedmonocarboxylic acids are particularly advantageous, among which acrylicand methacrylic acid are preferred. Examples of such monomers having twononconjugated ethylenically unsaturated double bonds are alkylene glycoldiacrylates and dimethacrylates, such as ethylene glycol diacrylate,1,2-propylene glycol diacrylate, 1,3-propyleneglycol diacrylate,1,3-butylene glycol diacrylate, 1,4-butylene glycol diacrylates andethylene glycol dimethacrylate, 1,2-propylene glycol dimethacrylate,1,3-propylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylateand 1,4-butylene glycol dimethacrylates, and divinylbenzene, vinylmethacrylate, vinyl acrylate, allyl methacrylate, allyl acrylate,diallyl maleate, diallyl fumarate, methylenebisacrylamide,cyclopentadienyl acrylate or triallyl cyanurate. In this context, theC₁-C₉-hydroxyalkyl methacrylates and acrylates, such as n-hydroxyethyl,n-hydroxypropyl or n-hydroxylbutyl acrylate and methacrylate, andcompounds such as diacetoneacrylamide and acetylacetoxyethyl acrylateand methacrylate are also of particular importance. Moreover, it is alsopossible organosilicon monomers of the general formulaRSi(CH₃)₀₋₂(OR¹)₃₋₁, in which R has the meaning CH₂═CR²—(CH₂)₀₋₁ orCH₂═CR²CO₂—(CH₂)₁₋₃, R¹ is a straight-chain or branched, unsubstitutedor substituted alkyl radical having 3 to 12 carbon atoms, which may beinterrupted by an ether group, and R² is H or CH₃. Examples of these arevinylmethyldimethoxysilane, vinylmethyidiethoxysilane,vinylmethyldi-n-propoxysilane, vinylmethyldiisopropoxysilane,vinylmethyldi-n-butoxysilane, vinylmethyldi-sec-butoxysilane,vinylmethyldi-tert-butoxysilane,vinylmethyldi(2-methoxyisopropoxy)silane andvinylmethyldioctyloxysilane. In the case of aqueous polymer dispersionsproduced exclusively by the method of free radical aqueous emulsionpolymerization, the abovementioned monomers are incorporated bypolymerization in general in amounts of from 0.5 to 10% by weight, basedon the total amount of the monomers to be polymerized.

The components of the redox system used for the process according to theinvention are expediently added simultaneously or in succession for theaftertreatment of the polymer dispersion heated to a temperature in therange from about 20 to 100° C., preferably from 25 to 90° C.,particularly preferably from 30 to 85° C., preferably under atmosphericpressure, but, if required, also at a pressure greater than or less than1 bar (absolute), while stirring. The redox system components accordingto the invention can be metered either after the end of thepolymerization or added in the form of one or more portions. The timefor the metering, the addition of one more portions and the time betweenthe additions depend on the chemical composition of the dispersion, thesize of the reaction batch, the reactor geometry and the half-life ofthe redox system used. Depending on the object, the duration of theaddition of the redox system may be from a few seconds to several hours.It has proven expedient to determine the duration in preliminaryexperiments. The sequence in the addition/metering/portion-by-portionaddition of the oxidizing agent and reducing agent is expedientlydetermined in preliminary experiments, all combinations beingpermissible in the process according to the invention. In a preferredembodiment, the oxidizing agent is added first and then the reducingagent is added.

The metering can be effected from above, from below or from the side.Particularly preferably, at least one component is either metered oradded in portions. Very particularly preferably, the reducing agent ismetered.

The metal salt solution can be added at various times during the redoxaftertreatment. The embodiment in which the reducing agent alreadycontains the metal salt is preferred.

The aftertreatment according to the invention is usually carried out ata pH in the range of less than or equal to 9. In principle, bases, suchas, for example, sodium hydroxide solution, ammonia water ortriethanolamine, can be used for adjusting the pH of the polymerdispersion. A pH range of from 2 to 9 is advantageous for theaftertreatment of the polymer dispersion, and a pH in the range between3 and 9 is preferred according to the invention.

Dispersions having viscosities of >100 mPa.s are advantageous for theprocess according to the invention, preferably dispersions of >1000mPa.s, particularly preferably dispersions of >2000 mPa.s, veryparticularly preferably dispersions of >5000 mPa.s, without asignificant decrease in viscosity taking place. The preparation ofaqueous polymer dispersions has frequently been described in the pastand is therefore known to a person skilled in the art [cf. e.g.Encyclopedia of Polymer Science and Engineering, Vol. 8, page 659 etseq. (1987)]. It is effected by emulsion polymerization of monomershaving at least one olefinically unsaturated group in the presence of apreferably water-soluble polymerization initiator and in the presence ofemulsifiers and, if required, protective colloids and customary furtheradditives. As a rule, the addition of the monomers by continuous feed iseffected here. The use of peroxodisulfuric acid and/or the salts thereofin amounts of from 0.1 to 2% by weight, based on the total amount ofmonomers, as initiator is preferred. The polymerization temperature isin general from 20 to 150 and preferably from 60 to 120° C. Thepolymerization takes place, if required, under pressure. In particular,anionic emulsifiers alone or as a mixture with nonionic dispersants inan amount of, in particular, from 0.5 to 6% by weight of the totalamount of monomers are used as emulsifiers.

The aftertreatment, according to the invention, of the aqueous polymerdispersion for reducing the amount of residual monomers is effected inparticular after at least 95% and preferably at least 98 to 99% byweight of the total amount of monomers in the free radical emulsionpolymerization have reacted.

Of course, it is possible to subject the aqueous polymer dispersionsintended for the aftertreatment to a physical process (e.g. inert gasand/or steam stripping) beforehand, simultaneously or afterwards. Thefree radical redox systems to be used according to the invention permitan effective reduction of the amount of residual monomers in arelatively short time.

The dispersions aftertreated with the redox system according to theinvention have, in an outstanding manner, particularly low residualmonomer and formaldehyde contents. In addition, they have no tendency toundergo yellowing.

The dispersions aftertreated with the redox system according to theinvention are therefore particularly suitable for the preparation ofcoating materials (inter alia paints, food coatings), adhesives (wood,paper, plastics films) and products for construction chemistry and forthe finishing of textiles and paper. The dispersions according to theinvention can also advantageously be converted by spray-drying intopowders having low VOC contents, which are used in products forconstruction chemistry and adhesives.

EXAMPLES

Preparation of the base dispersion for example 1:

A solution of 400 g of partly hydrolyzed polyvinyl alcohol having adegree of hydrolysis of 88 mol % (polyvinyl alcohol having a viscosityof 18 mPa.s as a 4% strength aqueous solution) in 3510 g ofdemineralized water was prepared in a 10 l glass stirred vessel reactorhaving an anchor stirrer, which was provided with feed facilities,reflux condenser, jacket heating and jacket cooling and a nitrogenconnection. After the addition of 5.5 g of anhydrous sodium acetate andof 3.5 g of antifoam (®Agitan 280, Münzing-Chemie) and flushing withnitrogen, 0.17 g of ®Rongalit in 12.8 g of water was added during theheating-up. At an internal temperature of 57° C., 350 g of vinyl acetatewere incorporated by emulsification. The internal temperature wasincreased to 60° C. and the polymerization was initiated by adding asolution of 0.16 g of tert-butyl hydroperoxide (70% strength, ®TrigonoxAW 70, Akzo-Chemie) and 12.8 g of water. At an internal temperature of67° C., the 3 hour linear and simultaneous metering of 3650 g of vinylacetate, an initiator solution 1 comprising 1.45 g of tert-butylhydroperoxide (70% strength, ®Trigonox AW 70, Akzo-Chemie) and 199 g ofwater and an initiator solution 2 comprising 1.54 g of ®Rongalit C and167 g of water was started. The jacket temperature was chosen so that,on gentle refluxing, the internal temperature increased to 80° C. andremained constant there during the entire metering time. After the endof the metering time, heating was continued for a period of one hour at80° C. The dispersion was then cooled and was used for all furtheraftertreatment experiments. The solids content was 52.5% and theviscosity 45000 mPa.s (Brookfield, spindle 6, 20 rpm, 23° C.). Theresidual monomer content of vinyl acetate was 7300 ppm.

Carrying out the aftertreatment:

Oxidizing agent a), reducing agent b) and, if required, Mohr's saltsolution were added to 1000 g of the dispersion in a 2 l round-bottomedflask at a temperature of 60° C. at intervals of 10 min. After theaddition of the last component, heating was continued over a period of45 min and the reaction was stopped with 1 ml of a 1% strength methylether hydroquinone solution in methanol. The batches were then cooled.The blank sample V1i was subjected to the same heating sequence,including the dilution with water by subsequent additions of redoxsystem, but without subsequent additions of redox systems.

The results are listed in the table below.

Example 1 with Comparative Examples

TABLE 1a Amounts of substances in the aftertreatment Weight of Weight ofred. taken Amounts of reducing agents Mohr's salt ox. taken Amountg/1000 g Sodium g (always g/1000 g of ox disp. sulfinate Sulfonatesulfite Total 10 ppm) in Example Oxidizing agent disp. mmol Reducingagent in 13 g H₂O mmol mmol mmol mmol 1.6 g H₂O 1 (acc. to ®Trigonox C¹1.00 5.15 ®Brüggolit FF06 1.24 3.71 0.774 3.20 7.68 0.01 the invention)V1a ®Trigonox C 1.00 5.15 Glyoxylic acid/bisulfite 1.54 7.697 7.70 0.01adduct*2 Na salt² V1b ®Trigonox C 1.00 5.15 Sodium sulfite 0.97 7.707.70 0.01 V1c ®Trigonox C 1.00 5.15 Glyoxylic acid/bisulfite 0.30 + 0.781.490 6.20 7.69 0.01 adduct*2 Na salt + sodium sulfite¹ V1d ®Trigonox C1.00 5.15 Sodium sulfite 0.97 7.70 7.70 0.01 V1e ®Trigonox AW 70⁴ 0.665.15 ®Brüggolit FF06³ 1.24 3.71 0.774 3.20 7.68 0.01 V1f H2O2 0.58 5.15®Brüggolit FF06 1.24 3.71 0.774 3.20 7.68 0.01 1b (acc. to ®Trigonox C1.00 5.15 ®Brüggolit FF06 1.24 3.71 0.774 3.20 7.68 0 the invention) V1g®Trigonox C 1.00 5.15 Ascorbic acid 1.36 7.72 0.01 V1h ®Trigonox C 1.005.15 Ascorbic acid 1.36 7.72 0 V1i ®Trigonox C 1.00 5.15 ®Brüggolit C⁵0.57 3.71 3.71 0.01 V1j Starting dispersion V1k ®Trigonox B⁶ 0.76 5.13®Brüggolit FF06³ 1.24 3.71 0.774 3.20 7.68 0.01V = Comparative examples¹®Trigonox C from Akzo Nobel is the trade name for tert-butylperbenzoate²Preparation of glyoxylic acid bisulfite adduct*2 Na salt: T. M. Olson,M. R. Hoffmann, J. Phys. Chem. 1988, 92, 4246--253³®Brüggolit FF06 from Brüggemann is the trade name for thetechnical-grade mixture of disodium 2-hydroxy-2-sulfinatoacetate in anamount of from 50 to# 60% by weight, sodium sulfite in an amount of from 30 to 35% by weightand disodium 2-hydroxy-2-sulfonatoacetate in an amount of from 10 to 15%by weight, based on the total weight of the mixture.⁴®Trigonox AW 70 from Akzo Nobel is the trade name for tert-butylhydroperoxide (70% strength in water).⁵®Brüggolit C from Brüggemann is the trade name for sodium formaldehydesulfoxylate dihydrate⁶®Trigonox B from Akzo Nobel is the trade name for di-tert-butylperoxide

TABLE 1b Results of the aftertreatment Viscosity Test Test Test mPa · sDecrease in Mohr's ppm vinyl ppm Yellowing after (Brookfield: spindle:viscosity Example Oxidizing agent Reducing agent salt acetateformaldehyde 3 weeks 6 /20 rpm) [%] 1 (acc. to ®Trigonox C ®Bruggolit FF06 yes 600 20 no 33700 −0.15 the invention) V1a ®Trigonox C Glyoxylicacid/bisulfite yes 7000 21 no 33200 −1.63 adduct*2 Na salt¹ V1b®Trigonox C Sodium sulfite yes 7000 22 no 32550 −3.56 V1c ®Trigonox CGlyoxylic acid/bisulfite yes 7000 22 no 32450 −3.85 adduct*2 Na salt +sodium sulfite V1d ®Trigonox C Sodium sulfite yes 7300 22 no 30900 −8.44V1e ®Trigonox AW 70 ®Brüggolit FF 06 yes 1200 12 no 29200 −13.5 V1f H2O2®Brüggolit FF 06 yes 5400 13 no 23150 −31.4 1b (acc. to ®Trigonox CBrüggolit FF 06 no 500 14 no 34400 +1.93 the invention) V1g ®Trigonox CAscorbic acid yes 700 34 yes 18950 −43.9 V1h ®Trigonox C Ascorbic acidno 1100 40 yes 19750 −41.5 V1i ®Trigonox C ®Brüggolit C yes 1200 59 no28000 −17.0 V1j Parallel blank test without redox treatment 7000 40 no33750 Ref. V1k ®Trigonox B ®Brüggolit FF06 yes 3800V = Comparative examples

These examples show that, in comparison with comparative examples(V1a-V1d), the byproducts of ®Brüggolit FF06 (glyoxylic acid/bisulfiteadduct*2 Na salt or sodium sulfite) which are not according to theinvention have no significant effect by themselves.

Examples 1 and 1b according to the invention exhibit no marked decreasein viscosity compared with the blank test, which however occurs incomparative experiments V1e and in particular V1f when water-solubleoxidizing agents are used.

Conventional water-soluble oxidizing agents have no particular effect(>1000 ppm residual monomer content) and ascorbic acid as a reducingagent leads to substantial yellowing after 3 weeks.

Example 2 and Comparative Example V2

A dispersion based on vinyl acetate/ethylene, having a viscosity of 7000mPa.s and a residual vinyl acetate content of 5100 ppm, was heated to atemperature of 60° C. In the aftertreatment, 0.14 part by weight, basedon dispersion, of ®Trigonox C or ®Trigonox AW 70 (70% strength) (0.14part by weight, based on dispersion) was added while stirring. After 10minutes, 0.17 part by weight of ®Brüggolit FF06 was added by metering asa 5% strength aqueous solution. After a further 10 minutes, 0.004 partby weight of a 1% strength aqueous Mohr's salt solution was added. TABLE2 Amounts of residual monomers in the aqueous polymer dispersion beforeand after the aftertreatment according to the invention (example 2) incomparison with a hydroperoxide (®Trigonox AW 70, comparative exampleV2). before after aftertreatment aftertreatment Viscosity [ppm] [ppm]change / % Vinyl acetate (Trig. C) 4700 70  −5 Ex. 2 Vinyl acetate 4700810 −15 V2 (Trig. AW 70)

Examples 3, 3b and Comparative Example V3

Ten kg of vinyl acetate/ethylene dispersion having a viscosity of 2500mPa.s and a residual vinyl acetate content of 2000 ppm were heated to atemperature of 60° C. In the aftertreatment, 10 g of ®Trigonox C (0.10part by weight, based on dispersion), 1.40 g of ®Trigonox 42S(tert-butyl peroxy-3,5,5-trimethylhexanoate) or 14.3 g of ®Trigonox AW70 (70% strength) (0.10 part by weight, based on dispersion) were addedwhile stirring. After 10 minutes, ®Brüggolit FF06 (200 g) was added as a5% strength aqueous solution (0.10 part by weight, based on dispersion).After a further 10 minutes, a 1% strength aqueous Mohr's salt solution(40 ml, 0.004 part by weight, based on dispersion) and 45 g of waterwere added. TABLE 3 Amounts of residual monomers in the aqueous polymerdispersion before and after the aftertreatments 3, 3b according to theinvention in comparison with a hydroxperoxide ( ®Trigonox AW 70,comparative example V3). before after aftertreatment aftertreatmentViscosity [ppm] [ppm] change/% Vinyl acetate (Trig. C) 2000 85 Example 3−7.7 Vinyl acetate (Trig. 42 S) 2000 41 Example 3b −3.2 Vinyl acetate(H₂O₂) 2000 900 Comp. −25 example V3

Example 4 and Comparative Example V4

10 kg of vinyl acetate/acrylate/VeoVa 10 dispersion having a viscosityof 200 mPa.s and a residual vinyl acetate content of 600 ppm were heatedto a temperature of 60° C. In the aftertreatment, 17.1 g of ®Trigonox C(0.17 part by weight, based on dispersion) or 24.4 g of ®Trigonox AW 70(70% strength) (0.17 part by weight, based on dispersion) were addedwhile stirring. After 10 minutes, ®Brüggolit FF06 (342 g) was added as a5% strength aqueous solution (0.17 part by weight, based on dispersion).After a further 10 minutes, a 1% strength aqueous Mohr's salt solution(40 ml, 0.004 part by weight, based on dispersion) and 49 g of waterwere added. TABLE 4 Amounts of residual monomers in the aqueous polymerdispersion before and after the aftertreatment according to theinvention (example 4) in comparison with a hydroperoxide  ®Trigonox AW70, comparative example V4). The residual contents for  ®VeoVa10 and theacrylate were <10 ppm in both cases. before after aftertreatmentaftertreatment after 7 days [ppm] [ppm] [ppm] Vinyl acetate (Trig. C)600 30 <10 Ex. 4 Vinyl acetate 600 80 80 V4 (Trig. AW 70)

Example 5 and Comparative Example V5

10 kg of vinyl acetate/acrylate/®VeoVa dispersion having a viscosity of200 mPa.s and a residual vinyl acetate content of 600 ppm were heated toa temperature of 60° C. In the aftertreatment, 34.2 g of a 50% strengthemulsion of ®Trigonox 21 S (tert-butyl 2-ethylhexanoate, 0.17 part byweight, based on dispersion), prepared from 17.1 g of ®Trigonox 21S,0.50 g of ®Hostapal BV (tri-tert-butylphenol EO sulfate, trade name ofClariant GmbH) and 16.6 g of water, or 24.4 g of ®Trigonox AW 70 (70%strength) (0.17 part by weight, based on dispersion) were added whilestirring.

After 10 minutes, ®Brüggolit FF06 (342 g) (0.17 part by weight, based ondispersion) was added as a 5% strength aqueous solution. After a further10 minutes, a 1% strength aqueous Mohr's salt solution (40 ml, 0.004part by weight, based on dispersion) and 49 g of water were added. TABLE5 Amounts of residual monomers in the aqueous polymer dispersion beforeand after the aftertreatment according to the invention (example 5) incomparison with a hydroperoxide (®Trigonox AW 70, comparative exampleV5). before after aftertreatment aftertreatment after 7 days [ppm] [ppm][ppm] Vinyl acetate 600 20 <10 Ex. 5 (Trig. 21 S in emulsion) Vinylacetate 600 80 80 V5 (Trig. AW 70)

In all examples, the aftertreatment was stopped after the end of theexperiment (total time from the first addition of the oxidizing agentwas 65 min) by means of a free radical scavenger (100 ml of a 1%strength hydroquinone monomethyl ether in methanol).

1. A process for reducing the amount of residual monomers in aqueouspolymer dispersions by chemical aftertreatment, comprising: treating anaqueous polymer dispersion containing residual monomers with a redoxsystem which comprises a) from 0.005 to 5% by weight, based on the totalweight of all monomers used for the preparation of the polymerdispersion, of at least one oxidizing agent based on an organic peroxidefrom the class consisting of the a1) peresters of the general chemicalformulaR¹—CO—O—O—R²,   (1a) in which R¹ and R², independently of one another,are substituted or unsubstituted alkyl, aryl, cycloalkyl, aralkyl, R³COor R³OCO, in which R³ is unsubstituted or substituted alkyl, aralkyl oraryl, and/or a2) percarbonates of the general chemical formulaR⁴—O—CO—O—O—R⁵,   (1b) in which R⁴ and R⁵, independently of one another,are unsubstituted or substituted alkyl, aryl, cycloalkyl, aralkyl, R⁶COor R⁶OCO, in which R⁶ is unsubstituted or substituted alkyl, aralkyl oraryl, it being possible for the radicals R⁴, R⁵ and R⁶ themselves tocontain percarbonates, and/or a3) perketals of the general chemicalformulaR⁷—O—O—CR⁹R¹⁰—OO—R⁸, (1c) in which R⁹ and R¹⁰, independently of oneanother, are hydrogen or alkyl and R⁷ and R⁸, independently of oneanother, are unsubstituted or substituted alkyl, aryl, cycloalkyl,aralkyl, R¹¹CO or R¹¹OCO, in which R¹¹ is unsubstituted or substitutedalkyl, aralkyl or aryl, and b) from 0.005 to 5% by weight, based on thetotal weight of all monomers used for the preparation of the polymerdispersion, of at least one reducing agent from the group consisting ofthe sulfinic acids and the salts thereof having the structureMO—SO—CR¹²R¹³R¹⁴   (2) in which M is hydrogen, NH⁴, a monovalent metalion or one equivalent of a divalent metal ion of the groups Ia, Ia, IIb,IVa or VIIb of the Periodic Table of the Elements, in which R¹²═OH,NR¹⁵R¹⁶, in which R¹⁵ and R¹⁶, independently of one another, arehydrogen or C₁-C₆-alkyl, in which R¹³=hydrogen or an alkyl, alkenyl,cycloalkyl or aryl group, it being possible for these groupds to have 1,2 or 3 substituents which, independently of one another, are selectedfrom C₁-C₆-alkyl, OH, O—C₁-C₆-alkyl, halogen and CF₃, in which R¹⁴═COOM,SO₃M, COR¹⁵, CONR¹⁵R¹⁶, COOR¹⁵, in which M, R¹⁵ and R¹⁶ have themeanings stated above, or, if R¹³ is aryl, this may be unsubstituted orsubstituted as stated above, R¹⁴ is also H, and the salts thereof. 2.The process as claimed in claim 1, wherein the redox system furthercomprises catalytic amounts of a polyvalent metal ion which may occur ina plurality of valency states.
 3. The process as claimed in claim 1,wherein peresters are used as oxidizing agents.
 4. The process asclaimed in claim 1, wherein percarbonates are used as oxidizing agents.5. The process as claimed in claim 1, wherein perketals are used asoxidizing agents.
 6. The process as claimed in claim 1, wherein2-hydroxyphenylhydroxymethylsulfinic acid or the sodium salt thereof,4-methoxyphenylhydroxymethysulfunic acid or the sodium salt thereof,2-hydroxy-2-sulfinatoacetic acid or the disodium or zinc salt thereof or2-hydroxy-2-sulfinatopropionic acid or the disodium salt thereof is thereducing agent.
 7. The process as claimed in claim 1, wherein the amountof oxidizing agent added is in the range of from 0.02 to 3% by weight,based on the total weight of all monomers, and wherein the amount ofreducing agent added is in the range of from 0.02 to 3% by weight,likewise based on the total weight of all monomers.
 8. The process asclaimed in claim 1, wherein the reducing agent comprises a mixture ofdisodium 2-hydroxy-2-sulfinatoacetate in an amount in the range of from50 to 60% by weight, sodium sulfite in an amount in the range of from 30to 35% by weight and disodium 2-hydroxy-2-sulfonatoacetate in an amountin the range of from 10 to 15% by weight, based on the total weight ofthe mixture.
 9. The process as claimed in claim 1, wherein the oxidizingagent and the reducing agent are fed in succession in separate feeds ascomponents for the aftertreatment of the polymer dispersion.
 10. Theprocess as claimed in claim 1, wherein at least one component is fed inby metering.
 11. The process as claimed in claim 1, wherein at least oneof the components is fed in portions.
 12. The process as claimed inclaim 1, wherein first the oxidizing agent and then the reducing agentare added.
 13. The process as claimed in claim 1, wherein the reducingagent is fed in by metering.
 14. The process as claimed in claim 2,wherein the metal ions for the aftertreatment of the polymer dispersionare added after the oxidizing agent and the reducing agent.
 15. Theprocess as claimed in claim 2, wherein the metal ions for theaftertreatment are added after the oxidizing agent and together with thereducing agent.
 16. The process as claimed in claim 2, wherein thepolyvalent metal ions used are iron ions.
 17. The process as claimed inclaim 1, wherein the temperature during the aftertreatment is in therange of from 20 to 100° C.
 18. The process as claimed in claim 1,wherein the aftertreatment is carried out under a pressure in the rangeof ≦1 MPa.
 19. The process as claimed in claim 1, wherein theaftertreatment is carried out at a pH in the range of from 2 to
 9. 20.The process as claimed in claim 1, which is carried out using a polymerdispersion having a viscosity in the range of greater than or equal to100 mPa.s.
 21. The process as claimed in claim 1, which is carried outusing a polymer dispersion which contains, as polymerizable monomers,esters of vinyl alcohol and monocarboxylic acids having from 1 to 18carbon atoms.
 22. The process as claimed in claim 1, which is carriedout using a polymer dispersion which contains, as polymerizablemonomers, esters of α,β-monoethylenically unsaturated mono- anddicarboxylic acids alkanols having from 1 to 12 carbon atoms, ornitrites of α,β-monoethylenically unsaturated carboxylic acids.
 23. Theprocess as claimed in claim 1, which is carried out using a polymerdispersion which contains, as polymerizable monomers, aromatic oraliphatic α,β-unsaturated, unsubstituted or halogen-substitutedhydrocarbons.
 24. (canceled)